In radiation therapy, a treatment planning system is often used to plan a treatment dosage for a patient. For high-energy photon radiotherapy, a three dimensional (3D) treatment planning system (TPS), for example using a pencil beam algorithm1-4, a convolution/superposition algorithm5-9 or a Monte Carlo algorithm10-14, may rely on a virtual mechanical and dosimetric representation of a treatment unit called the beam model to calculate dose on a 3D representation of the patient. The commissioning of a beam model includes determining beam model parameters that generate a reasonable fit between delivered and planned dose in non-clinical irradiation conditions. The measurements used for commissioning include beam profiles and percent-depth dose (PDD) curves or tissue-phantom ratios16 measured with an ion chamber and a scanning system in water. The evaluation of a beam model differs from dose calculation algorithm testing as it is performed in simple irradiation conditions (e.g., normal surface, rectangular fields and homogenous water phantom) and the observations can be fed back to the TPS to improve the agreement between planned and delivered dose. Published guidelines containing test procedures and acceptance criteria17-20 are available to assist the clinical physicist in the commissioning of the beam model.
The accuracy of the dose calculation on a 3D representation of a patient depends in particular on the type of dose algorithm used in the TPS and the accuracy in beam model commissioning. In an example TPS, such as the Pinnacle3, version 8.0h, from Philips Medical Systems (Madison, Wis.), beam model commissioning includes the use of a series of TPS auto-model scripts for a first-order fit followed by manual adjustments of various beam model parameters in a trial and error process. The manual optimization of a beam model during commissioning can be a time consuming task due to its iterative and trial-and-error nature. Furthermore, the quality of the beam model commissioning depends on the user's ability to manage multiple parameters and assess their various impacts on the agreement between measured and calculated dose.
With the advent of intensity modulated radiotherapy (IMRT), the requirements on measured data for TPS commissioning and beam model accuracy have heightened. For example, the impact of accurate beam penumbra measurement on beam model and IMRT patient-specific quality control (QC) performance has been demonstrated24, 25. For high-dose and high-precision radiotherapy such as stereotactic body radiotherapy (SBRT) of paraspinal tumors26-29, small geometric safety margins and high dose limits to critical organs at risk (OAR) stress the importance of beam model accuracy.